Crystallographic dependence of visible-light photoactivity in epitaxial TiO2-xNx anatase and rutile

Nitrogen-doped TiO2 materials have been shown to exhibit visible-light photoactivity, but the operative mechanism(s) are not well understood. Here we use structurally and compositionally well-defined epitaxial films of TiO2-xNx anatase (001) and rutile (110) (x <=similar to 0.02) to show a qualitative difference between the visible-light activities for the two polymorphs. Holes generated by visible light at N sites in anatase (001) readily diffuse to the surface and oxidize adsorbed trimethyl acetate while the same in rutile (110) remain trapped in the bulk. In light of the low doping densities that can be achieved in phase-pure material, conventional wisdom suggests that holes should be trapped at N sites in both polymorphs. Although the detailed mechanism is not yet understood, these results suggest that the hole hopping probability is much higher along the [001] direction in N-doped anatase than along the [110] direction in N-doped rutile.